An optical scanner is used to generate an electronic file, a bitmap file for example, which is representative of a scanned object such as a document or photograph. This is typically accomplished by passing a controlled light source across the surface of the object. Light reflects off the surface of the object and back onto an array of photosensitive devices such as a charge coupled device, or CCD, array. As the light source passes over the object, the CCD array converts the reflected light intensity into an electronic signal that is ultimately digitized into an electronic file once the entire object is scanned.
In a conventional flatbed scanner, the light source and CCD array are located in a base covered by a plane of transparent glass. The object being scanned is placed, or sandwiched, between the glass plane and a cover. The inside surfaces of some covers are constructed of a high reflectance white material. The high reflectance white surface enables the conventional scanner to reduce or eliminate dark borders around the document, black circles where punch holes exist, and dark borders around multiple images such as multiple receipts on a single scan. Moreover, the high reflectance white surface enables the conventional document scanner to improve the contrast of the document's image by reflecting the light that is transmitted through the object back to the CCD array. For example, when scanning a transparency, the light passing through the transparency reflects off the white cover and is detected by the CCD array.
Although a high reflectance white surface will allow a conventional scanner to eliminate unwanted dark areas, this white surface limits the ability of the conventional scanner. More specifically, scanners have the ability to detect the location of the object being scanned. This detection enables the scanner to provide electronic registration and electronic skew correction. Moreover, the detection of the location of the object's edges enables the scanner to provide automatic magnification selection. However, this edge detection depends upon the ability of the scanner to sense the difference in the reflectance between the object and the cover. Thus, some objects would better be scanned with a black rather than white background on the cover.
A scanner with this low reflectance background cover allows for reliable edge detection, but the same background fails to suppress the black borders or punch holes. Moreover, the low reflectance background provides very low contrast when attempting to scan objects such as transparencies or semi opaque objects.
Since there are problems with using just a high or a low reflectance background with a scanner, a scanner with at least two modes of reflectance is ideal. Liquid crystal display technology has been used for scanning systems to provide a set of reflectance options. These scanning systems offer a good solution to the problem, but they are expensive. Also, if a problem arises with the liquid crystals it is not easy or inexpensive to repair. In many cases it is less expensive to purchase a new scanner rather than attempting to repair a liquid crystal cover.